Asterisk / SIP-Call / AGI-Script / SIGHUP After Answer

I am facing a (for me) strange problem. When placing a SIP-Call I
normally get connected and the dialplan is executed. The Call-Flow is controlled by a PHP-Agi-Script. The script answers the call (via AGI-Command) and a simple voicefile is played. SOMETIMES(!) I get disconnected immediately after the Answer – without any reason. This happens about all fifth call.

Later on you will find my SIP-Debug-Output. I can see a “BYE”-Message. But why?

AGI-Debug-Messages:
(yes – I can the result is -1 > but why? Normally it is 0)